IVR has nothing to do with dash lamps. One thing in common with all of the dash lamps is the ground. Although there is a ground point for the IVR, often times the screws have to be inserted to hold the dash in place for the dash ground to work for the lamps.
A test for valid voltage to the lamps is to examine a lamp on the same circuit that is not located on the dash: clock, ignition, etc. If they work, then the problem is isolated to the dash cluster; if not working, the problem is elsewhere.
Here are a couple more questions for the dash wiring gurus.
I have a factory plug behind the instrument panel that has a blue wire with red stripe and a yellow wire with black stripe. The yellow wire seems to be hot when the accessory is on. The blue wire seems to be hot when the lights are on. What was this supposed to plug into? My only guess is maybe the original radio. I can’t find any kind of matching plug for it.
And then I have this pair of wires, one black, the other black with a green stripe, that come to a plug hanging in the center of the dash. Can anyone tell me to what this was supposed to connect?
First picture is the radio plug (power, dash lamps).
Second picture is PRNDL lamp lead.
Third picture I’m not as familiar with (I know Mustangs like the back of my hand—Cougar specific stuff not so much). Black/green wire is typically RUN-only power. That wire appears to be a male pin, so it must be receiving power from whatever plugs into it.
My current theory is that third photo is actually the mono speaker plug from the original AM radio. I won’t be able to confirm until I get the dash pad off because I can’t see where it goes past the HVAC stuff. Or I guess I could just stick radio speaker leads on it and see if music comes out.
That third pic is iffy. I just removed my dash speaker recently and it doesn’t match the wiring in my 68.
